I have given these issues some thought, Argosy24 has retrofitted his to have the seat belts.
The shoulder mounted seat belts can be done if you are adept at taking stuff apart and putting it back together. The main bow or rib that runs just behind the driver and co-pilot seats is a structural member. If you make a mounting plate for the upper portion of the belt and attach it to this rib all you need to do is find new belts. I would look at one of the big truck (semi) places so you can get one that is long enough to reach from that far back.
On the issue of airbags, I know of no aftermarket system. There is a plethora of sensors and wiring that SRS requires that would be way beyond the capabilities of most mechanics. Not to mention the liability if it does not deploy correctly of if it does deploy and injures someone. I smell lawsuit.
Besides the only thing that is bigger than we are is a semi anyhow, an Airbag is not going to make much difference then
